Central Interceptor enters next stage of construction

Auckland’s water utility Watercare is progressing with the
development of one of New Zealand’s most ambitious tunnelling projects, the
Central Interceptor. The project includes the construction of a 13 km
wastewater tunnel at depths of up to 110 m underneath the city, as well as 4.4
km of link sewers using microtunnelling.

The tunnel will run between Western Springs and the Mangere
Wastewater Treatment Plant, crossing Manukau Harbour approximately 15 m below
the seabed. The pipeline will connect to existing wastewater networks
along the route, which will divert flows and overflows into the tunnel.

As the CEO of Watercare Services, so my role is to make sure
the project is delivered to program and to budget. It is significant in the
sense that the project is costing about NZ$1.2 billion – a substantial sum for
us. It is by far the largest project we’ve ever done and possibly will ever do.

With very large projects, if you get variations or issues
with program delays, the sums of money you talk about are big. It may be a
small percentage, but it’s a small percentage of a big number. My job is to
make sure traditional project outcomes are delivered by the program director,
including scope, program, timetable and dollars.

More importantly, we’ve got to make sure the Central
Interceptor resonates and engages with our customers, and that we’re a good
neighbour during its construction, as we’ll be in locations throughout Auckland
for a while. This tunnel has a number of shafts located in built up residential
areas, so we don’t want to upset customers and stakeholders during the five
years of the project.

What does the project entail and why is it required?

This infrastructure was identified as being required over a
decade ago. It’s a large tunnel with an outside diameter of 5 m and inside
diameter of 4.5 m. It’s going to be 13 km long and will be up to 110 m deep.

The tunnel will pick up existing combined sewer networks and
take the flows to our largest wastewater treatment plant at Mangere, north of
Auckland airport on the southern side of the city.

In this part of central Auckland, wastewater and stormwater
flow into the same pipe. This system was designed that way at the turn of the
century with the intention these pipes would carry the stormwater to a nearby
stream, waterfront or coastal waterway where it would be discharged.

This was acceptable 100 years ago because dilution was
considered a means of treating sewerage; however, it’s now 2019 and for the
past few decades Auckland has been struggling with the decision to separate the
sewers and build new stormwater pipes.

Watercare is responsible for drinking water and wastewater services
in the Auckland region; however, stormwater is a council responsibility. Where
there’s a combined sewer, the stormwater goes into our wastewater system and
the law then requires us to address that issue.

The Central Interceptor was identified as a solution that
could be installed in a short period of time and address about 80 per cent of
the overflows as soon as it’s constructed. The project will also provide a
decade for Auckland Council to continue with separation works.

How was the project’s design developed?

The Central Interceptor is actually the spine of a larger
network, connecting to two link sewers which are substantial in themselves.
They’re not as large in diameter, but they will also be constructed using
trenchless technology.

The first challenge we had was the size of the pipe that we
need could have been substantially smaller, less than 2 m in diameter. But we
are building this tunnel in a built-up and older part of Auckland, and it’s
almost impossible to follow the grade and build it using traditional trenching
methods, so we had to go trenchless.

As soon as made this decision, we said, “Well, we’ve got to
go deep to stay away from the uncertain layers of the geology in Auckland.”

We also needed the diameter of the pipe large enough that it
would allow for good tunnelling. The advice we got was that we were therefore
looking at a diameter of more than 4 m.

In the end, we decided we would go for a larger pipe
diameter, with the larger capacity used to store storm flows. The Central
Interceptor reduces the need to increase the capacity at the treatment plant,
allowing us to transport and store, and at a diameter allowing for good
tunnelling.

The two link sewers will be constructed using
microtunnelling because they’re smaller in diameter, but the spine that is the
Central Interceptor will be constructed using a tunnel boring machine (TBM).

What did the tendering process involve?

Initially, there was an expression of interest, before we
shortlisted four consortiums, which subsequently submitted proposals. Any of
the four joint venture (JV) proposals could have done the job, but at the end
of the day, our criteria – in addition to price – specified experience and a
culture which reflected the values of Watercare.

We were looking for a contractor that would be a partner,
that would share our focus on our customers and make sure the project delivered
positive outcomes. In addition to the construction of the tunnel, over the next
five years the process is going to result in trained personnel, an enhanced
health and safety culture for workers and subcontractors, and positively
contribute to lowering carbon emissions, among other benefits.

The proposal from the Ghella-Abergeldie JV best met the
criteria we detailed in the contract, as well as Watercare’s aspirations. I’m
sure the other shortlisted groups would have done a fine job too, but the
process and the number of people who were involved and evaluated the bids
collectively decided the JV was the one for us.

After meeting the consortium’s personnel, the Ghella family
and the Abergeldie personnel, we are confident we made the right decision.

Is it important to provide the public with added value
when planning these largescale projects?

Absolutely, the community expects us to. It comes at a very
marginal cost, if any additional cost. It’s not a question of
‘either-or’, it’s ‘and’. You know, we can build the tunnel and have
other benefits because of the scale and the duration of the program.

The route of the Central Interceptor.

What’s the timeline for activities?

The JV got possession of the contract and the site in May
2019. The TBM will be fabricated, shipped, assembled, disassembled and then
sent to the receiver; it’s a process that’s going to take 12 months or more. In
the meantime, there’s a lot of preparation work to be done such as preparing
shafts for tunnelling.

Another significant undertaking is staffing, which the JV is
working on while waiting for the TBM to arrive.

When will the project be completed?

The tunnel will be commissioned in stages, so we won’t wait for
the entire length to be completed before we start to benefit from it.

The whole project will be completed at the end of 2024, but
we will be able to get the first section of the Central Interceptor up and
running by 2022 or 2023. As soon as it’s available and we see fit, we’ll start
using it.

Why has Watercare been using trenchless technologies over
traditional trenching in recent projects?

One of the key drivers is the fact that we’re building our
infrastructure in an area that is already built up and we want to minimise
disruption to our customers and roads, as much as possible. Going underground
allows us to do that.

New Zealand is a very young island from a geological point
of view, so it does present challenges for us, but I think the contracting fraternity
has consistently come back with innovative ways of doing the work and
cost-effectively. Even in the case of the Central Interceptor, the tenders have
come in, the contract has been signed and we will be within our budget, which
is a fantastic outcome.

Going underground means less destruction and the innovation
being demonstrated by the contractors has meant we haven’t paid a huge premium.
In fact, one of the water main jobs we’re doing now, where we’re avoiding going
through a commercial area, is going to be cheaper using trenchless technology
than if we had gone with open cut.

When we look at the cost, we don’t just look at the cost to
Watercare: we look at the cost to Watercare and to the community. Cost in terms
of money, businesses that may be disrupted because of the trenching work being
done in the streets, the non-financial costs in terms of noise and disruption
and dust and all the other things that come with construction.

We’ve found that we’ve got a lot of options when it comes to
addressing the challenges of building infrastructure. We select what makes
sense and trenchless technology has been very good to us.